Microbial carbon oxidation rates and pathways in sediments of two Tanzanian mangrove forests

Abstract

Temporal and spatial variations in benthic metabolism and anaerobic carbon oxidation pathways were assessed in an anthropogenically impacted (Mtoni) and a pristine (Ras Dege) mangrove forest in Tanzania. The objectives were (1) to evaluate how benthic metabolism is affected by organic carbon availability; (2) to determine the validity of diffusive release of CO2 as a measure benthic carbon oxidation; and (3) to assess the partitioning of anaerobic carbon pathways and factors controlling the availability of electron acceptors (e.g. oxidized iron). Microbial carbon oxidation measured as diffusive exchange of O2 and CO2 (32–67 and 28–115 mmol m−2 day−1, respectively) showed no specific temporal patterns. Low intertidal sediments at Mtoni fed by labile algal carbon of anthropogenic origin had higher diffusive CO2 release than high intertidal sediments that primarily received less reactive mangrove detritus. Diffusive release of CO2 apparently underestimated total sediment carbon oxidation due to CO2 loss from deep sediments via emission through biogenic structures (i.e. crab burrows and pneumatophores) and porewater seepage into creeks. We propose that diffusive fluxes in the present mangrove sediments are roughly equivalent to depth-integrated reactions occurring in the upper 12 cm. Anaerobic carbon oxidation was dominated by FeR irrespective of anthropogenic influence in sediments where the oxidizing effects of biogenic structures increased the Fe(III) level. More than 80% of the anaerobic carbon oxidation in Mtoni and Ras Dege sediments was due to FeR when reactive Fe(III) exceeded 30 μmol cm−3. The anthropogenic influence at Mtoni was primarily noted as up to one order of magnitude higher denitrification than at Ras Dege, but this process always accounted for less than 1% of total carbon oxidation. It is noteworthy that organic and nutrient enrichment of anthropogenic origin in Mtoni has no measurable effect on microbial processes, other than carbon oxidation in the low intertidal area and denitrification throughout the forest, and indicates a strong resilience of mangrove environments towards disturbances.